Using ansible, how would I delete all items except for a specified set in a directory

I'd do it like this, assuming a variable defined as 'managed_files' up top that is a list.

- shell: ls -1 /some/dir
  register: contents

- file: path=/some/dir/{{ item }} state=absent
  with_items: contents.stdout_lines
  when: item not in managed_files

Turns out it is possible to enter a host name directly into the playbook, so running the playbook with hosts: imac-2.local will work fine. But it's kind of clunky.

A better solution might be defining the playbook's hosts using a variable, then passing in a specific host address via --extra-vars:

# file: user.yml  (playbook)
---
- hosts: '{{ target }}'
  user: ...

Running the playbook:

ansible-playbook user.yml --extra-vars "target=imac-2.local"

If {{ target }} isn't defined, the playbook does nothing. A group from the hosts file can also be passed through if need be. Overall, this seems like a much safer way to construct a potentially destructive playbook.

Playbook targeting a single host:

$ ansible-playbook user.yml --extra-vars "target=imac-2.local" --list-hosts

playbook: user.yml

  play #1 (imac-2.local): host count=1
    imac-2.local

Playbook with a group of hosts:

$ ansible-playbook user.yml --extra-vars "target=office" --list-hosts

playbook: user.yml

  play #1 (office): host count=3
    imac-1.local
    imac-2.local
    imac-3.local

Forgetting to define hosts is safe!

$ ansible-playbook user.yml --list-hosts

playbook: user.yml

  play #1 ({{target}}): host count=0